1. Field of the Invention
The invention relates to a movement determination device for determining movement between two movable parts in a piston-cylinder unit.
2. Description of the Related Art
German reference DE 40 41 407 C2 describes a device for detecting the relative speeds of a piston and cylinder of a vibration damper. The device discloses a sound generator and a sound pickup arranged in the working space of the cylinder remote from the piston rod of the vibration damper. The bottom of the cylinder, specifically the bottom valve, serves as the reflection surface for reflecting the transmitted sound. Although this device is purported to be suitable for so-called single-tube vibration dampers, this assertion is difficult to confirm because single-tube vibration dampers have a separating piston that separates a compensation space from the working space remote from the piston rod. This compensation space--and thus the separating piston--must compensate for the volume of the in-and-out moving piston rod. The separating piston is movable in the axial direction of the vibration damper and therefore cannot be used as a reference mark. Furthermore, the separating piston is completely reflective, so that the cylinder bottom can supply no reflections or only too few reflections.
Another prior art device is disclosed in German reference DE 40 41 407 C2 in which a transmitter is located at the end of the piston rod. In this device, it is absolutely necessary to employ a hollow piston rod, so that the connection cable can be run to the sound generator and pickup. This creates additional design problems, because the piston rod must be hydraulically tight at its end. In addition, the contacts are also easy to assemble.
Yet another prior art device is disclosed by German reference DE 36 20 957 A1, which describes a suspension system with a fluid compression spring strut having a hollow housing supported by two end elements that are movable toward or away from each other in the axial direction of the spring. A device is provided on one end element to measure the axial distance between the end elements by the transmission of ultrasound pulses in the axial direction. The pulses are sent back by the other end element, for example, so as to measure the travel time of the pulses in the housing and thus determine the distance between the two end parts inside the housing at any time. Preferably, a transmitter-receiver is arranged on one end element and a reflector is arranged on the other end element, while a stationary reflector attached to the end element carrying the transmitter-receiver provides a reference value.
The ultrasound waves are propagated in the cushion of the spring. The gas inside the spring has practically no damping effect on the vibrating body, so that many signals are reflected from the surfaces in the spring, and the signal that represents the correct distance between the two end elements must be filtered out.
For physical reasons, this type of ultrasound transmission and reception in a gaseous environment is only able to correctly measure distances of at least 0.3 m between the end elements. Shorter distances can be detected only by separately operating transmitter and receiver parts. However, these entail considerable apparatus-related expense.